The Supreme Court of Canada declined appeals in two separate Alberta murder cases that had used the controversial defence.
By dismissing his appeal, the high court ordered a new trial for Michael Cairney of Edmonton, who was convicted of manslaughter in the Feb., 2007 shotgun slaying of his longtime friend, Stephen Ferguson.
Cairney argued that he was provoked into shooting Ferguson after discovering that Ferguson had been abusing and threatening his common-law wife.
A jury had earlier acquitted Cairney of second-degree murder and convicted him of manslaughter, but an appeal court later ordered a new trial.
In another case, the court upheld the second-degree murder conviction of Bill Pappas, who killed Brian Kullman in 2006.
Pappas had accused Kullman of extortion, and when he went to Kullman's home to confront him, Pappas said Kullman wouldn't back down and threatened his mother instead.
Kullman was killed with a handgun that Pappas had brought to the home.
A defence of provocation is sometimes used to reduce a murder charge to manslaughter.
Convictions for second-degree murder carry an automatic life sentence with parole possible in 10 to 25 years.
There is no minimum sentence for manslaughter.
But in a 5-2 decision Friday, the high court ruled that the Cairney case did not meet an "air of reality" test.
The test asks a jury to weigh whether an accused person acted in the heat of the moment, and whether any reasonable person could have been pushed to the snapping point by the victim's actions.
"There is nothing on the record to support the element of sudden shock required to cause an ordinary person to lose self-control," Chief Justice Beverley McLachlin wrote in the majority ruling.